You can't. A netfabb repair sends your model off to a cloud based server (an autodesk server as Netfabb is their software). They do the repair and send it back. It has nothing to do with your computers processing power.
Neophyil gave correct (simplified) explanation.. for more than that, a lot of data transfer analysis would be necessary (and probably some insight into netfabb code).
Imagine this scenario: you send file to netfabb, but whole job isn't actually done there. Quite possible netfabb sends chunks of (your) data back to you and instructs your PC what to do with that chunk of data. When that is done, your PC sends the result back to netfabb... and process repeats until full job is done. Yes, CPU time is precious even for Autodesk.
There exist (scientific) projects where everyone can participate by making PC (CPU actually) available for solving heavy calculations. After starting such application, you're asked how many (of available) cores you're willing to share and after that you start receiving chunks of data which is needed to be processed on your PC. Once it's finished, result is automatically sent back to server and you receive new packet of data.
Back to netfabb... maybe it can utilize only two cores...? I don't know the answer. Maybe you can ask on some netfabb specific forum and get more info there.
Its not even available on Windows 7, only Win 10. We are talking about the right click 'fix through netfabb option' the File>Repair Stl option is available still though. It was mentioned on Github, although I cant remember if it was on the release notes for a much older version or if it was mentioned in one of the raised issues. It was awhile a go.
I did try to adjust the Z leadscrew on my third failed netfabb experiement, unlike cura the ultimaker really didn't like it. (after that it decided it didn't like the 0,0,0 position and made a new one)
Yes its because of Windows version. The fix through Netfabb is only available when on Win 10. However you can use a normal browser to go to the netfabb website and if you have an autodesk ? account you can log in and repair models through the browser. Its just not as convenient.
Scripts are accessed through the Lua script library, located at Prepare > Run script > Run Lua script, where they are loaded, saved, edited, and run. A syntax checker and a debugger is available. All the main Lua commands can be accessed within the script. The includes the general Lua commands, however excludes the special Lua commands for 3S. Scripts do not have to work on content currently loaded in Netfabb and may in fact load, manipulate, and save an arbitrary FABBPROJECT file entirely independent from what is currently loaded for work by the user. Typically, however, a script is targeted at content already loaded. For this purpose, the variable tray is predefined with the current platform (and its LUATray object) which provides access to all meshes in the buildroom. In addition, the global variable netfabbapplication facilitates access to the trays of the currently running Netfabb application.